1. Field of the Invention
The present invention relates to a temperature control method for controlling the temperature of a hot plate in a heat processing apparatus for performing a heat process on a substrate placed on the hot plate, which is used in a coating/developing system for applying a resist coating onto a substrate, such as a semiconductor wafer, to form a resist film and then performing development on the resist film after light exposure. The present invention further relates to a temperature controller and heat processing apparatus, which are suitably compatible with the temperature control method.
2. Description of the Related Art
In the sequence of photolithography for semiconductor devices, a resist is applied onto a semiconductor wafer (which will be simply referred to as a wafer) to form a resist film. Then, the resist film is subjected to a light exposure process in accordance with a predetermined circuit pattern. Then, the light-exposed pattern thus formed is subjected to a developing process to form a circuit pattern on the resist film. During this sequence, various heat processes are performed, such as a heat process (pre-baking) after resist coating, a heat process (post-exposure-baking) after light exposure, and a heat process (post-baking) after development.
The resist coating, the development after light exposure, and the heat processes described above are preformed by a resist coating/developing system which includes a plurality of processing units for performing these processes and transfer units for sequentially transferring wafers to perform a series of processes.
In general, a heat processing unit for performing a heat process in this coating/developing system is structured to perform a heat process on a target substrate or wafer placed on a hot plate, which is provided with a heater and a temperature sensor built therein and is controlled to be at a target temperature by a temperature controller (for example, Jpn. Pat. Appln. KOKAI Publication No. 11-067619).
In a resist coating/developing system of this kind, heat processing units play a significant role, because the thermal history of a wafer determined by heat processes greatly affects the quality of devices. A thermal history determined by a heat process on a wafer is defined by an integrated amount of heat applied to the wafer by the heat process performed for a certain time period from loading of the wafer onto a hot plate to unloading of the wafer therefrom. Accordingly, in order to perform a heat process uniformly on a number of wafers so as to decrease the variations in device quality, the thermal histories of the wafers are preferably set constant.
In a heat process of this kind, when a semiconductor wafer is placed on a hot plate adjusted at a target temperature, the semiconductor wafer takes heat from the hot plate. The temperature of the hot plate is thus decreased temporarily, and then gradually returned to the target temperature. On the other hand, the temperature of the semiconductor wafer starts being gradually increased immediately after loading thereof onto the hot plate, then stabilized around the target temperature, and then decreased by unloading thereof from the hot plate.
Accordingly, in order to perform a heat process uniformly on a number of wafers so as to decrease the variations in device quality, the stable temperature times of the wafers, in which the temperature thereof is stable around a target temperature, are preferably set constant.
A heat process performed on wafers may be controlled by the time period from loading of a wafer onto a hot plate to unloading of the wafer from the hot plate. In this case, in order to make the stable temperature times constant, reaching times (temperature increase times) need to be set constant, wherein the reaching time is defined by a time period for increasing the temperature of a wafer from an initial temperature before staring the process to a temperature around a target temperature.
However, where a plurality of heat processing units are used for heat-processing a number of semiconductor wafers, individual differences, such as the heater capacity of a hot plate, are inevitably present between the heat processing units. Further, component parts and/or installation ambiences are not the same between the heat processing units. Consequently, the temperature increase times of the heat processing units differ from each other, and thus the stable temperature times of the heat processing units also differ from each other, thereby brings about variations in device quality among wafers.